Two main answers:
You can totally ignore it and use Ionic as before.
You can build your app in Stencil, and import Ionic components, for a much faster and smaller bundle.
One line about Stencil is that it offers 90% of what Angular offers, and is much faster. (Stencil has a router, which you can connect components with, but you don’t need to use it if you’re building standalone components.) Fair warning: I’ve temporarily stopped building in Stencil because I’m waiting for a bug with Cordova to be fixed. It’s pre-release for a reason.
The biggest difference in my eyes between Ionic 3 and 4 is that in 4 you don’t need to import everything. Maybe simplest to see this with ionicons. In ionicons 3, you need to import all the ionicons in order to use one. With version 4, you only need to import the ionicon you are using. Same thing is true with, say, ion-grid. You don’t need to import any other component.
Options (1) and (2) are opposite ends of a spectrum, maybe the simplest, but probably a lot of mixed strategies work too. To be determined.